In cellular telephone or other wireless voice or data systems, a served area is divided into cells, each of which may be further divided into sectors. Each cell may be served by a single base station, which is further connected to a message switching center (“MSC”), a subscriber management system (“SMS”), or a data router via a base station controller (“BSC”). A plurality of wireless communication devices/terminals are connected to the MSC, or the SMS, the router by establishing radio links with one or more nearby base stations.
In earlier cellular telephone technology, such as time division multiple access (“TDMA”), as a wireless communication terminal travels from one cell to another, the radio link between the terminal and the base station serving the first cell had to be broken and then replaced by a radio link between the terminal and the base station serving the second cell. In contrast, in a code division multiple access (“CDMA”) cellular telephone system, because the same frequency band is used for all cells and sectors, the first link need not be broken before connecting with the second link. Moreover, the CDMA waveform properties which provide processing gain are also used to discriminate between signals that occupy the same frequency band. A wireless communication terminal thus need not switch frequencies when a call is transferred from one cell or sector to another.
Multi-cell interferences in wireless systems occur when signals, either from the BSC or terminals, spill over to its neighboring cells and interfere with the operation of the neighboring cells' BSC or terminals therein. Consequently, the multi-cell interference is recognized as one of the major factors that impair the performance of wireless communication systems by limiting both the capacity and coverage of the wireless communication systems.
Various conventional methods for reducing multi-cell interferences that are known in the industry include applying a pseudo random noise (PN) offset, a frequency offset, and using directional antennas at the terminal. All of these methods try to separate the wireless signals in a particular cell from neighboring cells far apart to eliminate or reach a minimum tolerable interference level.
The PN offset method is mainly used in spreading spectrum communication systems. Although it does not reduce the interference signals, it spreads the interference signals over a wider spectrum and makes them behave like thermal noises. Using the PN offset method, the maximum isolation between neighboring cells varies depending on the system spreading gain.
The frequency offset method uses different frequencies for different cells. The major drawback is that it significantly reduces the spectrum usage efficiency since the implementation of that depends on the frequency reuse. For example, as it is understood in the industry, if a frequency reuse factor is chosen to be 3, the spectrum usage efficiency is also reduced by a factor of 3.
Finally, by arranging the directional antenna to point to a particular direction, signals from one particular cell will be enhanced while signals from other cells are suppressed. However, the directional antenna is traditionally bulky and expensive, and thus renders it impossible or impractical to equip the handheld wireless terminals therewith.
What is needed is an improved method and system for effectively reducing multi-cell interferences for achieving better communication quality.